+const TargetRegisterClass *TargetRegisterInfo::
+getCommonSuperRegClass(const TargetRegisterClass *RCA, unsigned SubA,
+ const TargetRegisterClass *RCB, unsigned SubB,
+ unsigned &PreA, unsigned &PreB) const {
+ assert(RCA && SubA && RCB && SubB && "Invalid arguments");
+
+ // Search all pairs of sub-register indices that project into RCA and RCB
+ // respectively. This is quadratic, but usually the sets are very small. On
+ // most targets like X86, there will only be a single sub-register index
+ // (e.g., sub_16bit projecting into GR16).
+ //
+ // The worst case is a register class like DPR on ARM.
+ // We have indices dsub_0..dsub_7 projecting into that class.
+ //
+ // It is very common that one register class is a sub-register of the other.
+ // Arrange for RCA to be the larger register so the answer will be found in
+ // the first iteration. This makes the search linear for the most common
+ // case.
+ const TargetRegisterClass *BestRC = 0;
+ unsigned *BestPreA = &PreA;
+ unsigned *BestPreB = &PreB;
+ if (RCA->getSize() < RCB->getSize()) {
+ std::swap(RCA, RCB);
+ std::swap(SubA, SubB);
+ std::swap(BestPreA, BestPreB);
+ }
+
+ // Also terminate the search one we have found a register class as small as
+ // RCA.
+ unsigned MinSize = RCA->getSize();
+
+ for (SuperRegClassIterator IA(RCA, this, true); IA.isValid(); ++IA) {
+ unsigned FinalA = composeSubRegIndices(IA.getSubReg(), SubA);
+ for (SuperRegClassIterator IB(RCB, this, true); IB.isValid(); ++IB) {
+ // Check if a common super-register class exists for this index pair.
+ const TargetRegisterClass *RC =
+ firstCommonClass(IA.getMask(), IB.getMask(), this);
+ if (!RC || RC->getSize() < MinSize)
+ continue;
+
+ // The indexes must compose identically: PreA+SubA == PreB+SubB.
+ unsigned FinalB = composeSubRegIndices(IB.getSubReg(), SubB);
+ if (FinalA != FinalB)
+ continue;
+
+ // Is RC a better candidate than BestRC?
+ if (BestRC && RC->getSize() >= BestRC->getSize())
+ continue;
+
+ // Yes, RC is the smallest super-register seen so far.
+ BestRC = RC;
+ *BestPreA = IA.getSubReg();
+ *BestPreB = IB.getSubReg();
+
+ // Bail early if we reached MinSize. We won't find a better candidate.
+ if (BestRC->getSize() == MinSize)
+ return BestRC;
+ }
+ }
+ return BestRC;
+}